$N^\ast$ Form Factors based on a Covariant Quark Model

TL;DR

This paper uses a covariant quark model to predict and analyze the electromagnetic form factors of various nucleon resonances, comparing with experimental data to understand the roles of valence quarks and meson cloud effects.

Contribution

It provides new covariant model predictions for $N^ ext{*}$ transition form factors and compares them with recent experimental data, highlighting the significance of quark and meson cloud contributions.

Findings

Good agreement with data for $Q^2 > 2$ GeV$^2$

Valence quarks dominate at high $Q^2$

Meson cloud effects are significant at low $Q^2$

Abstract

We discuss the results of the covariant spectator quark model for the several $\gamma^\ast N \to N^\ast$ transitions, where $N$ is the nucleon and $N^\ast$ a nucleon excitation. More specifically, we present predictions for the form factors and transition amplitudes associated with the resonances $N(1440)1/2^+$, $N(1535)1/2^-$, $N(1520)3/2^-$, $\Delta(1620)1/2^-$ and $N(1650)1/2^-$. The estimates based on valence quark degrees of freedom are compared with the available data, particularly with the recent Jefferson Lab data at low and large momentum transfer ($Q^2$). In general, the estimates are in good agreement with the empirical data for $Q^2 >2$ GeV$^2$, with a few exceptions. The results are discussed in terms of the role of the valence quarks and the meson cloud excitations for the different resonances $N^\ast$. We also review our results for the resonance $\Delta(1232)3/2^+$ and discuss the relevance of the pion cloud component for the magnetic dipole form factor and the electric and Coulomb quadrupole form factors.